This invention relates to visual display screens, and more particularly to display screens having a plurality of gas cells arranged across a plane in close capacitive proximity to electrical conductors. The selective application of voltages to the conductors causes gas ignition to occur in the cells, and further enables cell ignition to be transferred to adjacent cells under a unique combination of electrical signal timing and conductor physical spacing and geometry.
It has long been known that a gas cell can be fired into ignition upon the application of a suitable voltage across the cell. Neon lamps have used this phenomena to provide visual indicators driven by electrical circuitry. It has further been known that the separation of the voltage conductors from the gas cell by a dielectric medium such as glass causes gas cell ignition which fairly rapidly extinguishes when the free electrons in the cell accumulate along the inner dielectric surface to create an electric field opposing the field created by the voltage conductor. However, if the polarity is suddenly reversed across the voltage conductors, the dielectric electron accumulation acts in a voltage aiding sense with the field created by the reversed polarity voltages to cause a repeated cell ignition.
A 1962 publication in the Journal of Applied Physics entitled "Electrical Breakdown of Argon in Glass Cells with External Electrodes at Constant and at 60 Cycle Alternating Potential", by Bakkal and Loeb, describes this cell dielectric phenomena in considerable detail. The discovery disclosed by this publication is that, once sufficient voltage has been applied to initially ignite a gas cell, subsequent reversed polarity voltage pulses may be of lesser magnitude to sustain the cell ignition because of the additive effect created by the electron accumulation on the inner dielectric surface. This phenomena has been utilized to derive a number of prior art display devices, many of them utilizing gas cells arranged in a matrix with conductors, such as U.S. Pat. No. 2,933,648, Bentley; No. 2,925,530, Engelbart; No. 2,984,765, Engelbart; No. 3,340,524, Renauldi; and No. 3,559,190, Bitzer. Each of these patents disclose particular kinds of electrical drive systems for activating gas cells constructed according to the teachings of the Journal of Applied Physics publication.
A general understanding of the basic operation of a gas cell according to the phenomena discovered and disclosed in 1962 is necessary in order to fully understand the present invention. This phenomena presupposes a gas filled cavity having electrical conductors closely aligned and capacitively coupled, preferably by means of a glass dielectric. When the voltage potential between two lines disposed in close proximity to the gas filled cavity is made sufficiently high, a breakdown will occur in the gas space in the region immediately between the respective conductors and adjacent dielectric material. When this breakdown occurs the gas space contains movable charges, both electrons and positively charged ions, which are generated by the various physical processes responsible for the breakdown. The electrons move toward the most positive surface, and the ions move in the opposite direction towards the lowest potential surface. The electrons are by far the most mobile and therefore move with transit times of 10.sup.3 - 10.sup.4 less than the transit times of the ions. The electrons tend to accumulate on the dielectric surface over the positively charged conductor and the ions tend to accumulate on the dielectric surface over the negatively charged conductor.
The physical movement of these charges constitutes a current, and since the dielectric medium will not pass this current a voltage charge is developed by charge movement and accumulation. The accumulation of charges gives rise to a potential across the gas space which is developed in opposition to the applied voltage potential, and this opposition potential increases as the charges build up on the dielectric surface. This opposing potential eventually becomes high enough to extinguish the cell illumination, and since the electrons have much greater mobility their charge accumulation is primarily responsible for the cell extinguishing. Typical times required for sufficient electron accumulation to extinguish the cell are from 10.sup.-8 to 10.sup.-7 seconds, and at the point of cell extinction, the electrons have effectively been swept out of the gas space, whereas the positively charged ions have barely begun to move. The positively charged ions create a positive space charge in the gas space which will continue to migrate toward the negative voltage terminal if the voltage is continually applied to the voltage conductor. After a sufficient additional time the positive ions drift to the dielectric surface over the negative conductor and create a surface charge on this surface which is positive and in opposition to the negative voltage of the adjacent conductor. If the applied voltage is then removed, the respective positive and negatively charged dielectric surfaces maintain a field across the gas space in a direction which is opposed to the direction of the originally applied field. The magnitude of this field is the vector sum of the effect caused by the two oppositely charged dielectric surfaces.
If the voltage applied to the conductors is subsequently reversed in polarity it will be discovered that a much lower voltage magnitude is required to cause cell ignition than was the case in the first application of voltage to the conductors. This is because the subsequent reversed voltage polarity has the internal electric field, developed by the charges on the dielectric surfaces, acting in a voltage-aiding sense to cause a new gas breakdown and subsequent ignition. The foregoing process may be repeated with subsequent low voltage polarity reversals so that illumination of the cell is maintained under lower voltage parameters than were required for initial cell ignition. This phenomena has variously been referred to as the "wall charge" phenomena, cell "memory", and in other terms of art. The net result of the foregoing operation is that, for a given gas cell, the initial cell ignition potential requires the voltage of one predetermined magnitude and subsequent gas ignition potentials may be predetermined lower voltage signals.